Software Update scan failed

When I went to Software Update in System Preferences, it said 'Unable to check for updates". Then I went to terminal and type "softwareupdate -l", it shows me this:

Scan finished with error: Error Domain=SUMacControllerError Code=7301 "[SUMacControllerErrorScanNoUpdateFound=7301] scan for minor software update failed: [SUCoreError(Scan):kSUCoreErrorScanCatalogDownloadFailed(8400)_1_com.apple.MobileAssetError.Download:3]" UserInfo={NSUnderlyingError=0x7ffdec8063c0 {Error Domain=SUCoreError Code=8400 "[kSUCoreErrorScanCatalogDownloadFailed] failed to download catalog (MADownload domain result:3)" UserInfo={NSLocalizedDescription=[kSUCoreErrorScanCatalogDownloadFailed] failed to download catalog (MADownload domain result:3), NSUnderlyingError=0x7ffdec806430 {Error Domain=com.apple.MobileAssetError.Download Code=3 "Download failed. (Catalog download for com.apple.MobileAsset.MacSoftwareUpdate)" UserInfo={tryAgainLater=true, NSLocalizedDescription=Download failed. (Catalog download for com.apple.MobileAsset.MacSoftwareUpdate), generic=true}}}}, SUMacControllerErrorIndicationsMask=0, NSDebugDescription=[SUMacControllerErrorScanNoUpdateFound=7301] scan for minor software update failed: [SUCoreError(Scan):kSUCoreErrorScanCatalogDownloadFailed(8400)_1_com.apple.MobileAssetError.Download:3], NSLocalizedDescription=No available updates found. Please try again later.}

2020-08-19 08:57:12.421 softwareupdate[581:5444] Creating client/daemon connection: 0E97B910-129D-49CC-8594-6C826AF75501

No new software available.

Anybody helps me? Thanks a lot!!

P/s: macOS Big Sur beta 4, OpenCore 0.6.1